Friday, November 18, 2011

History and misuse of BMI

In response to yet another post over at SocImages, I delved even deeper into the coining of the term BMI and how it became used in the health world. I have previously written about BMI (here, here, and here), but I never really delved into how this unit had become a part of looking at population health (specifically obesity). Well, it was because I like participating in the commentary in SocImages that I started to look even further into the history of the thing. Using a lead from Wikipedia's entry on BMI, I found the 1972 document ("Indices of Relative Weight and Obesity") that first coined the phrase "body mass index" (the authors didn't even use the intialism in the article). I was pleased to note that the authors recognized that the body mass index should not be used to describe the individual:

What we here call the body mass index, W/H^2, has a long history. Because Quetelet was the first to calculate that ratio, W/H^2 has sometimes been called Quetelet’s index. But Quetelet himself did not actually advocate that ratio as the general measure of ‘build’ or of adiposity; he merely noted that in young adults W/H^2 was more stable than W/H^3 or W/H with increasing height. ... [No] proponents [prior to this paper] offered a convincing objective analysis in favor of the [Quetelet] index. Further, it should be observed that the greatest emphasis in almost all of the index making of the anthropometrists was on growth with relatively little consideration of the evaluation of body composition, nutritional status or adiposity.
...
As noted elsewhere [43], the use of ideal or recommended weight confounds age and weight because on the average weight increases with age until the fifties while increase in height is over by the early twenties at the latest. The general trend to continue growth in weight may be undesirable but it has no relevance to the question of providing an objective description of relative body mass; it is scientifically indefensible to include a value judgement in that description. The characterization of persons in terms of desirable weight percentage has resulted in attributing to ‘overweight’ some tendencies to ill health and death that are actually only related to age [43].

Of course, the disappointing thing about some of the commentators is that they don't understand that some terminology is quite old. The BMI was originally referred to as the Quetelet index (named after the man who conceived it), which was formulated in the early-middle 1800s; about 150 years before the paper that first coined "body mass index". Of course, some people apparently didn't know this. "EschewObfuscation" wrote (and at least 1 person "liked"):

But the whole purpose of BMI was to define fatness and thinness of a population. And why do that? So that doctors could talk to patients about their weight. Why? Because you shouldn't fall out side the norm or desired BMI number/category. Who decided normal? Who decided desired? How were those decisions made?

and

It is a social construction because the names we put on those numbers have social meaning. Why not call them "flower", "blue", "headphones", "towel" and "tricycle" instead of “underweight,” “normal,” “overweight,” “obese,” and “morbidly obese"? Or why provide categories at all and just use the number? Really what use is BMI anyway?

The words attached to numbers have social meaning that affects the society. Only one group is called 'normal' and that communicates a lot to the rest who don't fall in that category. Morbidly obese means "OMG!!! You are going to DIE!!!!11!!!!!!11" It is not a phrase that is purely objective.

Wow. That really did anything but eschewing obfuscation. It failed to approach and clear obfuscation in favor of allowing obfuscation to remain in order to justify what appears to be a normalized rationalization of what BMI is: an unjust, unobjective, and possibly something with an agenda to hurt people.

Now, it is, admittedly, a slightly difficult problem with regard to the idea of "objectivity", since its use for determining an individual's health condition is not objective, but many people fail to understand (quite apparently) the scientific use of BMI as it is used in public health research. My attempt to show why it is both an objective measure and a non-objective measure:

Also, BMI is "Completely NOT objective"? Remember, "objective" here means that it is not influenced by personal feelings, interpretations, or prejudice. Is measurement using standardized lengths (such as feet, meters, pounds, kilograms) not objective? Is dividing weight by the square root of the height (and applying the units correction of 703 if you did your measurements using feet and pounds) not give you an objective output? The measurement and calculation of BMI is objective.

However, is it useful for interpreting individual health conditions? No. (In this way it suffers from interpretive bias, is layered with social interpretation, has caused massive influence to personal feelings, and is - in this way only - not objective.)

In sum: BMI wasn't developed for doctors to talk to their patients about their weight. BMI wasn't used for studying obesity in a population until the 1970s (roughly 150 years after it was developed). BMI is a number calculated objectively based on objective measurements. BMI is next to useless in determining individual health conditions and suffers (for whatever reasons) a lot of social interpretations and personal feelings and is (in this final, falsely applied manner) not objective. Therefore, BMI is an objective, mathematical measurement and BMI - as it is come to be used - is non-objective social construction.

The blog entry itself is a great article about a book (How Much Do You Weigh?) that shows women of various different body types (heights and weights) and different BMI values. It can, hopefully, diminish the amount of negative pressure that surrounds the (ab)use of BMI in the public by showing women the vast panoply of what it means to be a "21" or a "27.4". (There was also a link in a comment by Tracy Rohlin to a posting at Jezebel that looks like a great website that does a similar thing.)

Now, in the end, do I think that BMI is an evil, unobjective, completely useless measurement? No, but that's not what it is supposed to be used for. At the same time, I don't think that BMI the best thing in the world for all population level analyses (nor the best thing in the world for population level analysis). However, it is a useful population measurement that can show trends over time, and looking at current, historical, and ongoing trends is a very important part of a lot of the sciences that we have. BMI is a next-to-useless metric to help all individuals understand - by the BMI alone - what their overall health condition is, not because it is a "bad thing," but because it is fundamentally not meant to be used in that manner. (Although it is probably a little more precise than one's daily horoscope.)

All that being said, the use of obesity to punish yourself or others is just wrong. Partly because it is based on a fundamentally flawed understanding of the application of a population level index, but mostly because using it to punish people is morally wrong.